1. Field of the Invention
This invention pertains generally to optical imaging devices and methods and more particularly to an apparatus and method for high-speed, real-time, two- and three-dimensional imaging enabled by optically amplified parallel to serial conversion. One embodiment of the method also uses active illumination of the object and provides a faster shutter speed, higher frame rate, and higher sensitivity than is capable in the art. Its applications include industrial inspection and monitoring, microscopy, and endoscopy both for industrial and medical uses.
2. Description of Related Art
The capability of real-time high-speed optical imaging is essential for capturing the evolution of dynamic events in a variety of systems and networks. For instance, in biomedical and clinical applications, there is an increasing need to study the transient dynamics of biomolecules. The time scale of these dynamic processes is usually on the order of nanoseconds or shorter. Likewise, achieving reliable real-time, high-speed optical microscopy is the key to studying and understanding ultrafast non-repetitive, transient chemical and biological processes such as molecular conformational changes, protein folding, apoptosis, myosin movement, and neural activity. Optical imaging with both high-speed and real-time imaging capabilities also allows high-throughput medical diagnostics such as cell-counting and the study of chemical signals between cells and within cells, and fault detection in the material testing industry.
Unfortunately, conventional detection techniques for imaging are slow and incapable of capturing dynamic processes that occur on the time scale of nanoseconds or shorter. This is due to the low frame rate of conventional image sensor arrays such as CCD and CMOS cameras in optical microscopes where the frame rate is typically 100 Hz-10 kHz. The speed is similar for the mechanically scanning laser scanners and for scanners where the microscope stage is scanned.
For CCD cameras, the low frame rate is partially due to the image download time. Because imaging requires data in two dimensions, downloading images from the CCD chip typically takes several milliseconds, while shutter speeds can be as short as 100 ns. In addition, conventional image sensor arrays such as CCD's and CMOS cameras suffer from individual element mismatches—a problem that limits the dynamic range of the system. These mismatches are particularly difficult to calibrate when fast single-shot detection is desired. The problem is similar to the interchannel mismatch problem which limits the dynamic range of multi-channel analog-to-digital converters.
Due to limitations in scan rate, conventional optical imaging techniques are inadequate for probing ultrafast events, especially for non-repetitive transient phenomena. This is due to the fundamental trade-off between sensitivity and speed. Higher sensitivity requires longer integration time and therefore lower speeds. Although an ultra-high-speed CCD with a frame rate of 1 Mfps (frames per second) has been reported recently, this was achieved by cooling the detector array to reduce the thermal noise. However, cooling is undesirable as it requires a refrigeration unit to accompany the camera. Another technique that was used to reach 1 Mfps was the use of a high intensity illuminator. This is undesirable for many applications due to the potential damage to the sample (particularly true for biological samples) as well as eye safety concerns for the user.
Another approach to imaging fast events is based on the so-called time-resolved pump-probe technique that has been used to capture dynamic events with temporal resolution down to picoseconds. The basic principle of typical pump-probe measurements is the following: A sample is exposed to a pump pulse from a light source, which generates some type of excitation or modification in the sample. After an adjustable time delay (controlled with an optical delay line), a probe pulse is directed to the sample, and its reflection or transmission is measured. By monitoring the probe signal as a function of the time delay, it is possible to obtain information on the decay of the generated excitation, or on other processes initiated by the pump pulses. However, the temporal resolution is fundamentally limited by the pulse duration. This technique has different variants which are used for different purposes, such as time-resolved coherent anti-Stokes Raman scattering (CARS) microscopy, X-ray diffraction imaging, pump-probe fluorescence microscopy, time-resolved magneto-optical Kerr effect (MOKE) microscopy, and pump-probe shadowgraph imaging. Typically, the time delay can be precisely adjusted by varying the relative path difference between the pump and probe. However, the fact that this is not a real-time imaging approach requires repetitive measurements at different time delays in order to acquire a complete sequence of images revealing the dynamics of the triggered event. In practice, the image acquisition speed is primarily limited by the relatively slow mechanical movement of the mirror which provides the tunable time delay (on the order of 1 kHz).
Accordingly, there is a need for a system and method for real-time, high-speed optical imaging and microscopy that not only provides two dimensional or three dimensional imaging but can also be scaled or sized for use in a variety of applications ranging from medical and non-medical endoscopy to barcode reading. The present apparatus and methods satisfy these needs, as well as others, and are generally an improvement over the art.